1. Field of the Invention
This invention generally relates to an optical waveguide hose having a fluid core, and more particularly, to an optical waveguide hose having a fluid core which is designed to prevent penetration of gas into the core so that the hose may maintain its function over a wide temperature range and a long period of time and which has high flexibility.
2. Prior Art
Prior art well-known fiber optics include optical fibers of inorganic glass systems such as quartz glass and multi-component glass and optical fibers of plastic systems such as polymethyl methacrylate and polystyrene. These are all solid optical fibers wherein both the core and the cladding are solid materials. Although these optical fibers have satisfactory characteristics, they are limited in flexibility because they are formed from glass or hard plastics. Also, when it is desired to increase the diameter for transmitting a large quantity of light, a plurality of optical fibers each having a diameter of about 10 to 1,000 .mu.m must be bundled. The optical fiber bundle has a space left among fibers even when fibers are packed at a possible maximum packing density and thus has a reduced effective inlet surface area for receiving light. The bundle is then loss efficient and rather expensive.
As one solution to the all solid optical fibers, the inventors proposed an optical waveguide hose using a normally liquid light transmitting medium in U.S. Pat. Nos. 4,009,382 and 3,814,497. The liquid system optical fiber includes a cladding in the form of a flexible hollow tube and a liquid core therein having a higher index of refraction than the cladding. Opposite end openings of the cladding are closed with window members. This allows the fiber to have a large diameter and a large effective light-receiving area and the fiber is thus highly efficient and cost effective.
Although the liquids system optical fiber had excellent features as mentioned above, it had the problem that since its stiffness is provided by a cladding in the form of a flexible hollow tubular member, gases can penetrate into the core liquid to lower its transparency due to a change in the service environment temperature during a long period of use. This is because the core is liquid and thus has a higher coefficient of expansion than the hollow tubular cladding generally formed of resinous material. At low temperatures, the volume of the core liquid is smaller than the interior volume of the cladding so that the hollow interior of the cladding is under negative pressure to allow gases to penetrate thereto through the cladding wall, creating bubbles in the core liquid. Bubble formation is facilitated particularly when the temperature is once elevated and then decreased.
UK Patent No. 1,450,608, for example, proposed a liquid system optical fiber which had solved the gas penetration problem. This optical fiber is provided with a core liquid reservoir connected to the hollow tubular cladding. The reservoir makes up the core liquid when the core liquid decreases its volume at low temperatures, preventing the hollow interior of the cladding from becoming negative in pressure and thus preventing gas penetration into the core liquid.
This optical fiber, however, required to form an aperture in the cladding for liquid communication before the reservoir could be connected to the hollow tubular cladding. This aperture caused light scattering and detracted from transparency. The attachment of the reservoir added to the weight and cost.